This invention relates generally to pressure vessels, and more particularly to repairing vessel cladding using a seal plate.
In a pressure vessel, for example a nuclear reactor pressure vessel, the reactor water environment is corrosive to unprotected reactor pressure vessel low-alloy steel. A stainless steel or nickel-base cladding normally forms a barrier to keep water from contacting the low-alloy steel. In cases where the cladding has been damaged causing a breach of the water sealing barrier, a repair must be considered, for example, by locally replacing or repairing the cladding in the defect area.
Direct welding repair of the welded cladding is typically performed using a temper bead welding technology, which permits welding on a hardenable material without the typical need for subsequent post-weld heat treatment to soften the underlying heat-affected zone (HAZ). Temper bead welding must be performed in a dry environment to provide sufficiently slow, controlled cooling rates as each weld pass is deposited. There is a risk of cracking the low-alloy steel during this operation, or later when returned to service, if the welding heat input is not properly controlled to result in a sufficiently soft HAZ microstructure to be able to withstand the inherently high tensile residual stresses of a welded repair on a heavy section, such as the vessel.
In addition to the cracking risks noted above, the specialized temper bead welding process requires precision placement of each bead to control its overlap on the adjacent, previously deposited bead in order to effect the correct degree of tempering without excessively softening or rehardening previously tempered portions of the HAZ. Such precision heat input control and bead placement is very difficult when the welding process is remotely applied, particularly when the access is severely limited as when servicing reactor vessel internal structures. In addition, the simple wet welding methods (where the arc is directly in the water) have not been developed for the application of existing temper bead technology. Further, application of any fusion welding techniques to low alloy steel in a damp or wet environment risks introduction of hydrogen-induced cracking.